A trip board is a board pivotally attached beneath the lower edge of a plow moldboard, e.g., the moldboard of a snowplow. A biased torsion spring situated between the moldboard and the trip board maintains the trip board in a generally coplanar relationship (or other desired relationship) with the moldboard. When the trip board strikes an unyielding obstruction on the roadway (or other surface to be plowed) during plowing, the torsion spring surrenders to allow the trip board to pivot backwardly so that the obstruction may pass beneath the trip board. When the plow blade passes over the obstruction, the trip board then pivots back to its original position. The trip board thus eliminates stress and impact damage to a moldboard of a plow blade by giving way when an obstruction is encountered. An exemplary trip board of this type is illustrated in U.S. Pat. No. 5,437,113 to Jones. Other types of trip boards using helical springs and air springs are also known to the art, as illustrated in U.S. Pat. No. 4,635,387 to Haring and U.S. Pat. No. 3,279,104 to Wandscheer et al.
Some difficulties are encountered with the installation and use of the aforementioned trip boards, more specifically, with the installation and use of the springs used to actuate the trip boards. Initially, the springs contact only small areas on the trip board (at the areas on the trip board abutting the spring ends), and therefore greater pressure is exerted on the trip board at these areas than at other areas. This leads to greater stress at these portions of the trip board, and additionally leads to an uneven distribution of biasing force over the length of the trip board. Additionally, while spring-loaded trip boards prevent moldboards from being significantly damaged when obstructions are encountered, their performance is less than ideal because they tend to propel moldboards into the air by as much as a foot or more upon impacting an obstruction. This effect, which arises owing to the elasticity of the springs, can be jarring to both the plowing vehicle and its driver. Further, the springs used in the trip boards are relatively expensive; they generally have an estimated usable life of only 1-3 years; and they are dangerous to install and remove because they must generally be preloaded before installation.